Today various luminescent materials are used for organic light emitting diodes, energy transducers or display elements. These luminescent materials are also used for means for preventing forgery of securities, means for inspecting contact surfaces and means for detecting trace amounts of substances.
In uses such as means for preventing forgery of securities, the luminescent materials are desired to be visually unnoticeable or colorless, but develop color and become easily visible when lighted in inspection.
Among such luminescent materials, organic materials which emit red light are conventionally known. However, most of such conventional red light emitting organic materials have a colored appearance.
In view of the problem, a method using a compound containing a europium (Eu) complex as a red light emitting organic material is disclosed.
For example, a recording liquid for inkjet using a fluorescent material comprising tetra-4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionato europium complex containing a tetra-n-butylammonium salt as a counter ion is proposed (see Patent Document 1).
The recording liquid is colorless under visible light, red at a wavelength of 600 to 650 nm under ultraviolet light having a wavelength of 254 to 365 nm, has high luminescence intensity and is excellent in light resistance.
Also, an ultraviolet-induced luminescent paste containing a europium complex using trifluoro-phenylbutanedione as a ligand is proposed (see Patent Document 2).
From its colorless appearance and emission of light by ultraviolet irradiation, the ultraviolet-induced luminescent paste is considered to have excellent visibility for determining the presence or absence of the paste.
Also, synthesis of europium-diketone chelate compounds and physical properties thereof are reported, and the report discloses that these compounds emit strong red light (see Non-Patent Document 1).    Patent Document 1: Japanese Patent Laid-Open No. 8-239609    Patent Document 2: Japanese Patent Laid-Open No. 2004-115559Non-Patent Document 1: Susumu Sato et al, “Some Properties of Europium β-diketone Chelates I (Synthesis and Fluorescent Properties)”, Japanese Journal of Applied Physics, Vol. 7, No. 1, pp. 7-13, January (1968)
However, most of the above-described conventional europium-diketone chelate compounds have high crystallinity. Such highly crystalline salts have poor processability such as coatability, film formability or dispersibility in a polymer substrate, and have problems with luminous properties such as uniformity and deterioration.
Also, while dispersibility in a polymer is required in some uses, the highly crystalline salts form particles and excellent dispersibility in a polymer cannot be achieved, often causing a problem of concentration quenching.
Moreover, the europium-diketone chelate compounds disclosed in Non-Patent Document 1 have an ionic bond, a low vapor pressure and melt or are oxidatively decomposed by heating even under reduced pressure, and therefore have a problem that the compounds cannot be formed into a film by vapor deposition.
The present invention has been made in view of the above problems and an object of the present invention is to provide a novel europium-diketone chelate compound which steadily emits red light by ultraviolet irradiation and a fluorescent material using the same.
Another object of the present invention is to provide a europium-diketone chelate compound having high dispersibility when applied to a glass substrate or mixed with plastics, and a fluorescent material using the same.